Microwave cavity having a varied impedance transmission line microwave energy seal around the access door opening



May 12, 1970 J. R. WHITE MICROWAVE CAVITY HAVING A VARIED IMPEDANCE TRANSMISSION I LINE MICRQWAVE ENERGY SEAL AROUND THE Filed Feb. 16-, 1968 AGCES S DOOR OPENING 2 Sheets-Sheet J.

W MICROWAVE SOURCE 23 FIG. 5

I 9 INVENTOR.

JEROME R. WHITE war/yaw.

ATTORNEY May 12, 1970 J R. WHITE 3,511,959

MICROWAVE CAVITY HAVING A VARI'ED IMPEDANCETRANSMISSI ON LINE MICROWAVE ENERGY SEAL AROUND THE. I

' I ACCESS noon OPENING Filed Feb. l6, 1968 FIG. 7

INVENTOR. 76 JEROME R.WHITE 74 Mf/Ztifl 73 v v ATTORNEY 2 Shet-Sheet 2 nited States Patent U.S. Cl. 219-1055 13 Claims ABSTRACT OF THE DISCLOSURE Two parallel spaced apart conductive rectangular bars are mounted at each of the four sides of a rectangular access door opening of a microwave heating cavity parallel to the edge of the door opening on the side of the cavity wall facing the conductive access door. The surfaces of the bars facing the access door are covered with a layer of polypropylene dielectric material. In the closed position, the access door rests against the covered bars. The bars have widths of about one-quarter wavelength of the microwave energy applied to the cavity in the medium between the bars and access door, and are spaced apart about one-quarter wavelength of the applied microwave energy in the medium between the access door and the cavity wall in the space between the bars. The access door is hinged at the cavity wall at a distance from the access door opening of about one wavelength of the applied microwave energy in the medium between the access door and the cavity wall structure.

BACKGROUND OF THE INVENTION Microwave cavities for subjecting materials to electromagnetic energy are provided with doors for inserting and removing articles to be heated, performing maintenance inside the cavity, and, in general, for allowing access to the interior of the cavity as desired. In providing microwave cavities with access doors, it is particularly important to place a microwave energy seal about the door opening which limits the amount of microwave energy leakage around the door to at least below the accepted safe level of milliwatts per square centimeter. Various techniques have been used in the prior art to seal the access door openings of cavities. However, such techniques have not always satisfactorily sealed the access door openings.

For example, seals depending upon an electrical contact between the surfaces of the door and cavity structure entirely about-the door opening have been employed to prevent undesirable microwave energy leakage around the door. Such sealing structures generally have not been effective because inadvertent gaps often exist in the electrically contacting surfaces about the door opening whereby undesirable microwave energy is allowed to leak to the surroundings. Also, arcing generally occurs which rapidly deteriorates the seal. To overcome these problems, various structures have been devised to provide sufiicient pressure to effect good electrical contact between the electrically contacting surfaces. Some of the structures are described in the U.S. Pats. 2,958,754, 3,296,356 and 3,304,360. Such structures are generally complicated making them difficult to clean. Since good electrical contact, hence, minimum leakage and arcing, depends upon the cleanliness of the electrically contacting surfaces, it is necessary that the surfaces be maintained clean. In industrial applications where cavities are in constant use, the maintenance required to keep the electrically contacting surfaces clean is expensive.

3,511,959 Patented May 12, 1970 To overcome the limitations and disadvantages of the prior art electrically contacting seal structures, cavities have been constructed to have open-ended quarter-wavelength and short-circuited half-wavelength chokes formed between the access door and the cavity structure about the door opening. Examples of some of these structures are described in the U.S. Pats. 2,632,090, 3,197,600, 3,210,512 and 3,249,731. Such microwave energy seals have the advantage of not depending upon the existence of electrically contacting surfaces to form the seal. However, they do depend upon the formation of an extremely low impedance, preferably approximating zero, between the access door and cavity wall about the edge of the door opening. Unfortunately, the presence of losses in the chokes cause the impedance about the edge of the door opening to be considerably larger than the theoretical value of zero, thereby allowing undesirable amounts of microwave energy to escape from the cavity.

Other techniques have been suggested to solve the problems of undesirable leakage and arcing. For example, U.S. Pat. 2,500,676 describes a microwave energy seal combining both electrically contacting surface tech niques with microwave choke techniques to accomplish the desired seal. U.S. Pat. 3,182,164 describes a twoconductor transmission line-type seal having a cavity formed in one of the lines. Sealing structures such as described in these patents are inconvenient to clean and large, generally requiring thick and space-consuming doors.

Therefore, considerable advantage is to be gained by providing a microwave cavity with an access door opening having a microwave energy seal thereabout which prevents arcing and limits the amount of microwave energy leakage around the door to a level beneath that which is undesirable. Additional advantages will be realized by providing such a microwave energy seal whose structure is compact and easy to clean, and suitable for use with both sliding and hinged access door structures.

SUMMARY OF THE INVENTION The present invention relates to microwave cavities for subjecting materials to electromagnetic energy. More particularly, it relates to a microwave cavity with an access door having a varied impedance transmission line microwave energy seal structure around the access door opening to limit the leakage of microwave energy about the access door.

Accordingly, it is an object of this invention to minimize the leakage of microwave energy from a microwave cavity.

More particularly, it is an object of this invention to limit the leakage of microwave energy from about the access door opening of a microwave cavity.

Another object of this invention is to permit easy and rapid cleaning of the microwave energy seal structure of microwave cavities.

A further object of this invention is to provide an access door structure for preventing the undesirable escape of microwave energy from the microwave cavity which is compact and easy to assemble.

Still another object of this invention is to provide a microwave energy seal structure for access doors of microwave cavities which prevents the undesirable escape of. microwave energy from the cavities by creating a noncontacting R.F. short circuit about the edge of the access door opening.

Yet a further object of this invention is to provide a microwave energy seal structure suitable for use with both sliding and hinged-type access door structures of microwave cavities.

The present invention is a microwave cavity characterized by the foregoing and other objects and advantages to thereby overcome the limitations and disadvantages associated with the prior art microwave cavities. In accordance with the present invention, an access door of conductive material is mounted spaced from a portion of the conductive wall of a microwave cavity defining the access door opening to form a transmission line structure around the access door opening having an open origin end proximate the edge of the opening and a terminal end distal the edge. Means are interposed between the access door and wall portion of the cavity defining the opening so that the transmission line has a plurality of series connected sections, each of which has an electrical length equal to about an odd multiple of a quarter wavelength. The sections beginning at an odd number of quarter wavelengths from the origin end of the transmission line structure have :a characteristic impedance greater than those of the immediately preceding and succeeding sections. The transmission line sections beginning at the origin of the transmission line structure and at about an even number of quarter wavelengths therefrom have an end distal the origin end which is open. The terminal end of the transmission line structure is terminated with means to reflect a low impedance, preferably, approaching zero, at the origin end of the transmission line structure.

'When the transmission line structure has an even number of sections, the terminal end of the structure is terminated with an impedance smaller than the characteristic impedance of the terminal transmission line section, preferably formed by electrically contacting surfaces around the opening defining a RF. short circuit. As is explained in detail hereinbelow with reference to the figures, the differences in the characteristic impedances of the sections reflects a much lower impedance at the origin end of the transmission line structure than that at the terminal end. By reflecting a lower impedance at the origin end, leakage and arcing is reduced significantly below that which would exist in the absence of the varied impedance transmission line structure. Hence, gaps which often are present in the electrically contacting surfaces are less harmful in the seal structure of the present invention than those of the prior art structures.

The varied impedance transmission line structure also can be constructed with an odd number of sections. In such cases, the terminal end of the structures would be terminated with an impedance greater than the characteristic impedance of the terminal transmission line section, preferably, formed by an open circuit at the terminal end. As is explained in detail hereinbelow, the varied impedance transmission line structure of the present invention reflects a much lower impedance at the origin end of the transmission line structure than do the prior art quarter wavelength choke structures which have a uniform characteristic impedance. Hence, the leakage of microwave energy through the seal of the present invention is far less than that through the prior art seal structures.

Furthermore, from the foregoing it becomes apparent that the structure of the microwave energy seal of the present invention is less complex and easier to clean than the more complicated structures discussed hereinbefore which form microwave energy seals. Because of this feature, microwave cavities having microwave energy seals constructed in accordance with the present invention are uniquely suited for industrial applications where maintenance is an important operating cost factor.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects and advantages of the present invention will become more apparent from the following detailed description and appended claims considered together with the accompanying drawings in which:

FIG. 1 is a perspective view of one embodiment of the microwave cavity of the present invention.

FIG. 2 is a perspective view of a portion of another embodiment of the microwave cavity of the present invention.

FIG. 3 is a schematic illustration of the electrical circuit formed by an open-ended embodiment of the varied impedance transmission line microwave energy seal employed in the microwave cavity of the present invention.

FIG. 4 is a schematic illustration of the electrical circuit formed by a short-circuited terminal end embodiment of the varied impedance transmission line microwave energy seal employed in the microwave cavity of the present invention.

FIG. 5 is an enlarged cross-sectional view taken along line 5-5 of FIG. 1 illustrating one embodiment of the microwave energy seal of the present invention.

FIG. 6 is an enlarged plan view of a portion of the embodiment of the microwave energy seal of FIG. 2 delineated by line 66.

FIG. 7 is an enlarged cross-sectional view taken along line 7-7 of FIG. 6.

FIG. 8 is a cross-sectional view taken along line 8--8 of FIG. 1 illustrating an embodiment of the microwave cavity of the present invention having an access door hinged from and electrically contacting the cavity wall portion defining the door opening.

FIG. 9 is a cross-sectional view of an embodiment of the microwave energy seal of the present invention for providing a seal about an :access door opening of a microwave cavity at two frequencies.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIG. 1, the microwave cavity 11 of the present invention comprises a plurality of metallic wall members 12, for example, of aluminum, joined together as by welding to define a chamber 13 for subjecting materials to electromagnetic energy. The microwave cavity 11 is excited by a microwave generator 14 operated to provide microwave energy at a selected frequency and energy level. The generator 14 is coupled to the cavity 11 by a microwave transmission line 16 and a transmission line feed 17 located at one of the wall members 12.

In accordance with the present invention, one of the wall members, such as front wall member 12', is pro vided with an access door 18 so that, for example, materials can be inserted and withdrawn from the chamber 13. In the embodiment illustrated in FIG. 1, the access door 18 is hinged at the front wall member 12 (see FIG. 8). However, detached sliding access doors 18, such as illustrated in FIG. 2, can be employed as well.

Referring to FIG. 3 and 4, undesirable leakage of microwave energy from around the access door 18 is prevented by a varied impedance transmission line structure 19 formed by a conductive panel 21 of the access door 18 mounted spaced from the facing surface 22 of the conductive cavity wall member 12' defining the access door opening 23 (See FIG. 5). The transmsision line structure has an electrical open origin end 24 proximate the access door opening 23 and either an electrical open (FIG. 3) or an electrical R.F. short (FIG. 4) at its terminal end 26 proximate the edge 27 of the conductive panel 21 of the access door 18. In either terminating case, the transmission line structure 19' comprises a plurality of sections (in FIG. 3, 28a, 28b and 280, and in FIG. 4, 28d and 28a) having an electrical length equal to about an odd multiple of a quarter wavelength, preferably, each about one quarter wavelength long. The sections of the transmission line structure 19 beginning about an odd number of quarter wavelengths from the origin end 24, i.e., section 2812 in FIG. 3 and section 28:: in FIG. 4, are constructed to have a greater characteristic impedance than that of the immediately preceding section, i.e., sections 28a and 28d of FIG. 3 and 4 respectively, and, when existing, that of the immediately succeeding section, i.e., section 28a of FIG. 3. This change in the characteristic impedance along the transmission line structure 19 can be accomplished by various means such as by changing the dielectric material, hence, the dielectric constant between the transmission line conductive members 21 and 22 along the transmission line structure, by changing the spacing between the conductive members 21 and 22, or by changing both.

Considering the embodiment schematically depicted in FIG. 3, the output end of the quarter wavelength transmission line section 280 is open and defines the terminal end 26 of the transmission line structure 19. Losses that are present in the section 280 form a virtual finite impedance at the terminal end 26 which is greater than the characteristic impedance of the transmission line section 280. This virtual finite high impedance is reflected as a low impedance at the junction 29 of the transmission line sections 280 and 28b. The impedance reflected at junction 29 is given by the equation:

where Z is the impedance reflected at junction 29.

Z is the virtual impedance representing the losses in the transmission line section 280 at the terminal end 26, and

Z is the characteristic impedance of the transmission line section 280.

This low impedance, Z is reflected as a high impedance at the junction 31 of the transmission line sections 28b and 28a. Because the characteristic impedance of the section 2812 is larger than that of section 280, the impedance reflected at junction 31 is higher than the virtual impedance, Z at the terminal end 26 formed by the open output end of section 280. The impedance reflected at junction 31 is given by the equation:

where Z is the impedance reflected at junction 31, and Z is the characteristic impedance of the transmission line section 28b.

From Equation 2, it is found that the impedance, Z reflected at junction 31 is greater than the virtual impedance, Z at the open terminal end 26 of the section 280 by the factor of the square of the ratio of the characteristic impedance, Z of section 28b and the characteristic impedance, 223g, of section 280.

This high impedance, Z at the junction 31 is reflected as a low impedance at the input end of the transmission line section 28w which defines the origin end 24 of the transmission line structure 19. This reflected low impedance is given by the equation:

Z24: any 28a) 9 where Z is the impedance reflected at the origin end 24 of the transmission line structure 19, and Z288v is the characteristic impedance of the transmission line section 28a.

ture 19. The reflected impedance, Z is reduced by a factor of the square of the ratio of the characteristic impedance, Z285, of section 28a and the characteristics impedance, Z of the higher characteristic impedance section 28b.

Because of the lower impedance, Z reflected at the origin end 24 of the transmission line structure, less microwave energy will escape from around the access door 18 of the cavity 11 to the surroundings. Although FIG. 3 illustrates a three-section varied impedance transmission line structure 19, a greater odd number of transmission line sections with the sections starting an odd number of quarter wavelengths from the origin end 24 having a larger characteristic impedance than their adjacent sections can be employed to form the microwave energy seal. The additional sections will effect a further reduction of the impedance reflected at the origin end 24 of the structure 19. In practice, it has been found that a transmission line structure 19 having at least three sections limits the microwave energy leakage to a level below that considered hazardous. However, the varied impedance transmission line structure is like a slow wave structure operated in its stop band. Therefore, the longer the electrical length of the structure 19, the less important the nature of the termination of the structure becomes in determining the sealing effectiveness of the structure 19. Varied impedance transmission line structures 19 having more than five seriallyconnected sections will function as an effective microwave energy seal regardless of the termination.

The desirable low impedance, Z also can be formed at the origin end 24 of the transmission line structure 19 by terminating the structure 19 with an R.F. short circuit. More specifically, referring to FIG. 4, the short circuit embodiment of the transmission line structure 19 includes an even number of sections, such as 28d and 282, each having an electrical length of about a multiple, preferably of one, quarter wavelength. The output end of the transmission line section 282 distal the origin end 24 of the structure 19 defines the terminal end 26 and is provided by an R.F. short circuit means 32. In practice, the R.F. short circuit means 32 can be formed by electrically contacting the conductive access door 18 to the conductive front wall 12' about the entire periphery 33 (see FIGS. 1 and 2) of the door 18. The impedance Z formed by the R.F. short circuit means 32 is reflected as a high impedance Z at junction 34 of-the transmission line sections 28d and 28e. This high impedance Z is reflected as a low impedance Z at the origin end 24 of the structure 19. This reflected low impedance is given by the equation:

where Z is the impedance reflected at the origin end 24 of the transmission line structure 19,

Z is the characteristic impedance of the transmission line section 28d,

Zzg is the characteristic impedance of the transmission line section 28a,

Z is the impedance formed at the terminal end 26 of the short circuited transmission line section 28e.

As is found by examining the Equation 4, the impedance Z reflected at the origin end 24 of the structure 19 is reduced from that of the short circuited terminal end 26 by a factor of the square of the ratio of the characteristic impedance Zggd and Z of the transmission line sections 28d and 28e. The lower impedance Z reflected at the origin end 24 of the structure 19 and the varied characteristic impedance feature of the structure 19 reduce the leakage which occurs if gaps exist in the R.F. short circuit means 32 and the tendency to arc at the R.F. short circuit means 32. A greater even number of transmission line sections with the sections starting an odd number of quarter wavelengths from the origin end 24 having a larger characteristic impedance than their adjacent sections can be employed to form the microwave energy seal. Although the additional sections effect a further reduction of the impedance reflected at the origin end 24 of the structure 19, tWO sections are sufiicient for most applications.

Referring now to FIG. 5, an open-ended-type of varied impedance transmission line structure 19 is illustrated as employed in the cavity 11 of FIG. 1 to provide a microwave energy seal about the periphery of access door 18. The transmission line structure 19 includes first and second conductive rectangular bar members 34 and 36 secured as by welding to the surface 22 of the cavity wall member 12' around the access door opening 23. The width of each of the rectangular bars 34 and 36 in the plane of the cavity wall member 12 is about one quarter wavelength of the microwave energy applied to the cavity 11. The rectangular bars 34 and 36 are spaced apart in the direction of their widths a distance of about one quarter wavelength of the applied microwave energy to define a space 37 therebetween. The transmission line structure 19 is completed by mounting the access door 18 so that the conductive panel 21 does not contact the conductive bars 34 and 36 when in the closed position. To insure that the conductive panel 21 does not contact the conductive bars 34 and 36, preferably a low loss dielectric material such as polypropylene 38 is interposed between the bars 34 and 36 and panel 21. In the illustrated embodiment, polypropylene coating or tape is placed on the bars 34 and 36. However, polypropylene could be placed on the conductive panel 21 as well.

The varied impedance transmission line structure of FIG. defines three-quarter wavelength sections such as described and shown in FIG. 3, sections 28a and 28c being the same. The conductive bars 34 and 36, the facing surface portions of the conductive panel 21, and interposed polypropylene dielectric tapes 38 define the transmission line sections 28a and 28c. The facing surface 22 of the cavity wall member 12, the facing surface portion of the conductive panel 21 and the space 37 define the transmission line section 281). An embodiment of the transmission line structure 19 employed in a microwave cavity 11 excited by source 14 operated at a frequency of 915 mHz. to provide 25 kw. of power is constructed as follows. The width dimensions, w, of the rectangular bars 34 and 36 are 3 inches. The thickness dimensions, t, of the bars 34 and 36 are 1 /2 inches. The thickness of the polypropylene dielectric tapes 38 are inch. The distance between the bars 34 and 36, or width of spacing 37 is 3% inches. The difference in the width of the spacing 37 is compared to those of the conductive bars 34 and 36, hence, length of the transmission line section 28b as compared to the sections 28a and 28c, principally is due to the different dielectric medium of the section 28b. Because of this difference in the dielectric medium, the wavelength of the applied microwave energy in section 28b is different from that in sections 28a and 280. The effective length of the sections also is lengthened electrically by the capacity due to the fringing fields at the junctions of the transmission line sections where the characteristic impedance of the structure 19 changes. In practice, the effective dielectric constant and thickness of each section are modified empirically to obtain an electrical length of a quarter wavelength in the environment defined by the section. This modification can be as large as 50% of that predicted by theory.

The characteristic impedance, Z of the transmission line section 28b is about 60 times the characteristic impedances, Zzg and Z of the identical transmission line sections 28a and 28c. The microwave energy escaping around the access door 18 is well below mw./cm. The ratio of the characteristic inipedances can be selected as desired, and may be altered by changing the thickness of the dielectric medium between the facing conducting surfaces of the structure 19 or by changing the dielectric medium. The characteristic impedance of each of the transmission line sections of the structure 19 is approximately inversely proportional to the square root of the dielectric constant of its dielectric medium and directly proportional to the thickness of its dielectric medium.

FIGS. 6 and 7 show an alternate arrangement of the varied impedance transmission line structure, for example, as employed in the microwave cavity 11 embodiment of FIG. 2 having a detached sliding access door 18. In that embodiment, the microwave energy seal is formed by a transmission line structure 19' having five sections 41a, 41b, 41c, 41d, and 41e, section 41a defining an open origin end 42 proximate the edge 43 of the access door opening 44, and section 41c defining an open terminal end 46 at the edge 47 of the'access door 18. The transmission line sections 41a-e are constructed to prevent leakage around the access door 18 with the cavity 11 excited from a source operated at 2450 mHz. The structure 19' has three parallel spaced rectangular conductive bar members 48, 49 and 51 secured to the surface 52 of the metallic front wall member 12 by nuts 53 and bolts 54 around the entire access door opening 44. Each bar member has a width dimension, w, of 1% inches (corresponding to about one-quarter wavelength in air of the microwave energy applied to the cavity 11), and thickness dimension, t, of inch. Low loss polypropylene dielectric rectangular bar members 56 and 57 are sandwiched between the conductive bar members 48, 49 and 51. The polypropylene bar members 56 and 57 are fixed in place by spring loaded pins 58 carried by the conductive bars 48, 49 and 51 to seat in pin-receiving recesses 59 and 61 at opposite sides 62 and 63 of the polypropylene bar members adjacent to the conductive bar members.

Each of the polypropylene bar members 56 and 57 have a thickness of inch so as to extend A inch beyond the conductive bar members 48, 49 and 51. When the access door 18 is closed, its conductive surface 64 facing towards the front wall member 12 rests against the respective facing outer surfaces 66 and 67 of the polypropylene bar members 56 and 57. In this manner, the

larger thickness polypropylene bar members 56 and 57 serve to hold the access door 18 spaced out of electrical contact with the conductive bar members 48, 49 and 51 when the door is in its closed position. Using polypropylene bar members to hold the access door of cavity 11 away from and out of electrical contact with the conductive bar members has several advantages over the polypropylene tape embodiment of FIG. 5. Most importantly from an industrial application standpoint, the polypropylene bar member embodiment has a longer life and requires less maintenance.

The width of each of the polypropylene bar members 56 and 57 is l inches, which is less than those of the conductive bar members 48, 49 and 51. This corresponds to an electrical length of about one-quarter wavelength of the microwave energy applied to the cavity in the dielectric medium of polypropylene. Although the length of the transmission line sections 41b and 41d including the polypropylene bar members is less than the sections 41a, 41c and 41e including the conductive bar members, as explained hereinbefore with reference to the embodiment of FIG. 5, this difference is due to the difference in the dielectric medium of the sections.

In the embodiments illustrated in FIGS. 57, the sections of the varied impedance transmission line structures 19 and 19' are formed by mounting conductive bar members and low loss dielectric material to the front wall member of the cavity 11. However, the transmission line structure of the present invention can be formed in other ways. For example, the conductive bar members and low loss dielectric material could be mounted to the conductive access door panel, for example, '21 in FIG. 5, or 52 in FIGS. 6 and 7, facing the front wall member of the cavity 11. Alternatively, the conductive bar members could be removed and either the front wall member or access door of the cavity 11 undulated to form the transmission line section. Also, the conductive bar members could be interposed between the access door and front wall member mounted insulatingly apart from both by, for example, embedding the bar members in a plastic dielectric material fixed to either the access door or wall member.

The particular varied impedance transmission line seal embodiments described hereinbefore have been those constructed to provide a microwave energy seal at a particular frequency or multiples or submultiples thereof. However, often it is desired to use a microwave cavity at various frequencies which are not exact multiples or submultiples of a particular desired frequency. In such cases, plurality of varied impedance transmission line structures could be serially connected to form a multiple frequency sealing structure. In such an embodiment each of the transmission line structures would be constructed to function as a microwave energy seal at a particular frequency.

In some applications, particularly industrial ones, it is desired to use the microwave cavity at different frequencies one of which, although not an exact, is a close multiple of the other frequency. In industrial applications, the microwave frequencies of 915 mHz. and 2450 mHz. are used. FIG. 9 illustrates a varied impedance transmission line microwave energy seal structure 19" particularly suited for such applications. This structure is a dual frequency microwave energy seal, i.e., provides a seal at two distinct frequencies. The dual frequency seal structure 19" comprises two conductive bar members 81 and 82 mounted to the front wall member 12' of the cavity 11 respectively proximate the access door opening 23 and the edge 27 f the access door 18. Both of the widths of the conductive bar members 81 and 82 are about one-quarter wavelength at the lower frequency, for example, 915 mHz., or about 3 inches, in the medium between the bar members 81 and 82 and the access door 18. A third conductive bar member 83 is mounted to the front wall member 12' between and spaced from the conductive bar members 81 and 82. The widths of the spacings 84 and 86 between the bar members and the width of the bar member 83 is about one-quarter wavelength at the higher frequency, for example, 2450 m'Hz. or about 1% inches, in the medium between the access door 18 and front wall member 12' in the spacings 84 and 86. Because the quarter wavelength at the frequency of 915 mHz. closely approximates three times the quarter wavelength at the frequency of 2450 mHz., the structure 19" appears as a transmission line having five sections when the cavity 11 is excited at a frequency of 2450 mHz. The two sections defined between the access door 18 and the conductive bar members 81 and 82 are about three-quarters of a wavelength long. The section defined between the access door 18 and the conductive bar member 83 and the two sections defined by the spacings 84 and 86 between the bar members are about one-quarter of a wavelength long. When the cavity 11 is excited at a frequency of 915 mHz., the structure 19" appears as a transmission line having three sections, each of. which is about onequarter wavelength long. Two sections are defined between the access door 18 and the conductive members 81 and 82. The other section is defined by the region between the conductive bar members 81 and 82 and the access door 18. Although the conductive bar member 83 exists in this region, its width is sufliciently small compared to a quarter wavelength at 915 mHz. so that region between the bar members 81 and 82 appears as a high characteristic impedance transmission line section.

To maintain the access door 18 out of electrical contact with the bar members 81, 82 and 83, each of the surfaces of the bar members facing the access door is provided with inch polypropylene dielectric coating 87.

In any case, to effect the desired microwave energy seal about the access door of a microwave cavity in accordance with the present invention, it is only necessary to form a plurality of series-connected transmission line sections about the access door opening of the cavity, with each having an electrical length equal to an odd multiple of a quarter wavelength and with those beginning at an odd number of quarter wavelengths from the origin end of the transmission line structure having greater characteristic impedance, and terminate the structure so that a low impedance is reflected at the origin end.

In the microwave heating cavity 11 embodiment of FIG. 2, a detachable sliding access door 18 is employed to close the access door opening 44. To hold the access door 18' in place, L-shaped dielectric guides 68 of, for example, polypropylene, are secured by nuts 69, bolts 71 and bolt plate 72 to the front wall member 12' at the bottom and each side of the access door 18'. However, with reference to FIGS. 1 and 8, the access door 18 also could be held in place by a hinge 73 mounted directly to the front wall member 12' along the bottom edge 74 of the access door 18. If the hinge 73 is of conductive material, an RF. short circuit would be formed at the point where it is joined to the front wall member 12'. As described with reference to FIG. 4, to insure that a low impedance is reflected to the origin 24 of the transmission line structure 19, the hinge 73 is secured to the conductive wall member 12' at a location 76 equal to an even number of quarter wavelengths from the origin 24 of the transmission line structure 19.

What is claimed is:

1. Apparatus for subjecting materials to microwave energy comprising conductive walls forming a chamber for subjecting materials to microwave energy, at least one of said walls defining an access door opening, said wall defining said opening having a wall portion surrounding the opening forming a transmission line element, an access door mounted for movement to a position closing said opening and including a conductive panel overlapping said access door opening and forming another transmission line element spaced from and facing said surrounding wall portion transmission line element when said access door is in said closed position, said overlapping conductive panel and surrounding wall portion defining a transmission line structure with an origin end proximate the access door opening and a terminal end distal said opening, said transmission line structure comprising a plurality of series connected sections each of which has an electrical length equal to an odd multiple of about a quarter wavelength of the microwave energy delivered to the chamber at selected frequencies, and means productive of a higher characteristic impedance in the transmission line sections beginning at an odd number of quarter wavelengths from said origin end than in the adjacent series connected sections.

2. The apparatus according to claim 1 wherein termination means is provided at said terminal end of the transmission line structure productive of reflecting a low impedance relative to the impedance of the termination means at the origin end of said transmission line structure.

3. The apparatus according to claim 2 wherein said transmission line structure comprises an odd number of series connected structures each of which has an electrical length equal to an odd multiple of about a quarter wavelength, and said termination means comprises means to hold said conductive panel spaced from the surrounding wall portion at the terminal end of said transmission line structure to define an RF. open circuit at the terminal end.

4. The apparatus according to claim 2 wherein said transmission line structure comprises an even number of series connected structures each of which has an electrical length equal to an odd multiple of about a quarter wavelength, and said termination means comprises means to form an electrical connection at the terminal end of said transmission line structure to define an RF. short circuit at the terminal end.

5. The apparatus according to claim 1 wherein said characteristic impedance productive means comprises parallel spaced bars of conductive material interposed said wall portion surrounding said access door opening and said conductive panel of said access door overlapping said wall portion to be spaced from at least one of said access door and said wall portion, each of said bars of a selected thickness in the direction of a line extending between the access door and wall portion, each of said conductive bars having a width dimension in a plane parallel to that of the planes defined by the access door and wall portion equal to an odd multiple of about a quarter wavelength of the microwave energy delivered to the chamber at selected frequencies in the space between the conductive bars and spaced transmission line elements, each of said spaces between said conductive bars being an odd multiple of about a quarter wavelength of the microwave energy delivered to the chamber at a selected frequency in the spaces between the spaced conductive bars with one of said conductive bars at the origin end of the transmission line structure.

6. The apparatus according to claim 5 wherein said conductive bars are rectangular in cross section and have widths equal to about one-quarter wavelength of the microwave energy delivered to said chamber at a selected frequency.

7. The apparatus according to claim 5 wherein said conductive bars are joined to and in electrical contact with one of said transmission line elements defined by said access door and said wall portion and spaced from the other transmission line element.

8. The apparatus according to claim 7 further comprising low loss dielectric material interposed said conductive bars and the spaced transmission line element.

9. The apparatus according to claim 8 wherein said low loss dielectric material is polypropylene fixed to said conductive bars.

10. The apparatus according to claim 7 further comprising bars of low loss dielectric material sandwiched 12 between said conductive bars and having a greater thickness than said conductive bars to engage said spaced transmission line element and prevent its contact with said conductive bars.

11. The apparatus according to claim 5 wherein at least three parallel spaced conductive bars are interposed said access door and said wall portion, said conductive bar on each side of the parallel spaced bars having a width equal to about one-quarter wavelength of the microwave energy delivered to said chamber at a first selected frequency, and at least one of said conductive bars between said conductive bars located on each side of the parallel spaced bars having a width equal to and spaced from adjacent conductive bars an electrical distance equal to about one-quarter wavelength of the microwave energy delivered to said chamber at a second se lected frequency higher than said first selected frequency.

12. The apparatus according to claim 1 wherein said access door has a hinge joining it to said wall defining said access door opening, said hinge forming an RF. electrical contact between said access door and wall defining said access door opening, and said hinge is joined to said wall defining said access door opening at a distance equal to an even multiple of about a quarter wavelength from the origin end of said transmission line structure.

13. The apparatus according to claim 1 further comprising a source of microwave energy coupled to said heating chamber to provide selected microwave power at a selected frequency.

References Cited UNITED STATES PATENTS 2,500,676 3/1950 Hall et al. 219-10.55 3,219,747 11/1965 McAdams. 3,351,730 11/1967 Pahlman 219-1055 DARRELL L. CLAY, Primary Examiner US. Cl. X.R. l7435; 333-98 

